It is generally recognized that many explosive tools have reduced performance when operating at higher wellbore pressures, particularly above 15,000 psi, when compared to when operating at lower wellbore pressures. The commercially available explosive cutters, for example, work reasonably well at lower hydrostatic pressure, say, below 15,000 psi, but are often marginal or ineffective above that pressure. See U.S. Pat. Nos. 7,146,913 and 6,644,099 to W. T. Bell. Explosive shaped charge cutters and severing tools are often used for critical situations where drill pipe is stuck and needs to be cut and pulled, while an expensive drilling rig is sitting idle, and this becomes more difficult at high pressure.
Perforating guns are also similarly affected by high hydrostatic pressure, as illustrated in FIG. 1. The test series for the FIG. 1 data was performed in a pressure vessel using steel encased Berea sandstone targets. The sandstone was open to the well pressure so that pore pressure in the sandstone was the same as the well pressure. There is a general decline in penetration with increased well pressure, with an apparent 10 percent or so decrease between 4,000 psi and 16,000 psi. The adverse effect of high wellbore pressure is also documented in a paper by Berhmann and Halleck, SPE 18243 “Effects of Wellbore Pressure on Perforator Penetration Depth”, 1988.
Higher pressure situations are becoming more frequent in the field and conventional approaches to design are limited in mitigating the effect. For cutters, for example, the traditional approach is to optimize the design by trial and error by increasing explosive, liner shape and density, initiation, which might squeeze out a marginal improvement in cutting at higher pressures. This effort is handicapped by the increased temperature requirements that typically accompany higher pressure, necessitating using an explosive which has inherently lower output, such as HNS. In addition, high pressure requires thicker walled and higher strength tubulars and drill pipe, as well as thicker walled housing for the explosive cutter, making a successful operation more difficult to achieve.
We recognize that if the hydrostatic pressure can be reduced temporarily during the short operating time required for an explosive cutter to initiate and complete the cut, that the cutter's performance could be increased, with an effectiveness that is comparable to operation in a lower pressure environment. One way to do this is to open a volume downhole prior to initiating the cutter. A volume opened quickly reduces the fluid pressure surrounding the cutter at the time of detonation, creating a temporary lower effective hydrostatic pressure for a better cut.
A similar situation occurs with perforating guns when operated at high hydrostatic pressures. The formation rock in these situations often has high compressive strength which can further reduce penetration. Shaped charge manufacturers can mitigate this somewhat with designs that are tailored to the high strength rock, but the wellbore pressure effect would still be present. Again, a temporary lowering of pressure in the wellbore surrounding the gun at the time that the gun initiates could increase the resulting penetration in the formation.
Conventional perforating gun systems sometimes use an empty chamber or gun volume that creates a dynamic underbalance. Although, sometimes effective in removing perforating damage through surge flow, the underbalance occurs too late to affect the penetration process itself: the shaped charge jet still has to penetrate through the high pressure fluid in the wellbore.
Perforating guns that have shaped charges with liners that contain reactive materials may be particularly susceptible to this same effect because the bulk of the reactive products are in the trailing slug and arrive inside the perforations at a later time than the jet that produces the perforation. See Bell, M. R. G., Hardesty, J. T., Clark, N. G. “Reactive Perforating: Conventional and Unconventional Applications, Learnings and Opportunities”, SPE122174, SPE European Formation Damage Conference, Netherlands, 27-29 May 2009. The effectiveness of these types of charges require that both the jet and the slug reach the perforation, meaning that there is more time for interference to occur and making the charge performance more susceptible to interference at high hydrostatic pressures.
The background above describes problems in performance of explosive devices at high hydrostatic wellbore pressure. Implicit in the understanding of the effect is that a lower hydrostatic pressure can alleviate some of the problem. Our invention couples explosive devices with existing implosion devices in a unique way to counter much of the wellbore pressure effect by a transient reduction of that pressure at the time of firing an explosive device. Our invention can also be used to control or modify the transient pressure at some remote position in the wellbore. Unregulated transient pressures from initiating an explosive device such as a perforating gun can upset plugs or packers, for example. By properly timed initiation of an auxiliary implosion or explosive device, the net transient pressure at a plug can be significantly reduced.
There are several applications for transient control of pressure down hole by opening a chamber to the surrounding fluid. One was suggested in the 1980s to create a controlled implosion for a downhole seismic application. See U.S. Pat. No. 4,805,726 to D. Thomas Taylor et al. Later, the idea was expanded to improve operations by inducing a dynamic underbalance during perforating, allowing better perforation cleanup by creating a favorable differential pressure between the formation and the wellbore for a short period of time. Later, this idea was incorporated into U.S. Pat. No. 6,598,682 to Ashley B. Johnson et al. In more recent years, implosion chambers have been used successfully to clean up existing scaly perforations by the surge created when a chamber is suddenly opened. See Harive, Kevin, Le, Cam, Khalek, Mohamed Abdel, “Service for Dynamic Scale Removal of Barium Sulfate in Perforation Tunnels,” SPE 143244, SPE European Formation Damage Conference, Netherlands, 7-10 Jun. 2011 and Busaidy, Adil Al, Zaouali, Zouhir, Baumann, Carlos Erik, Vegliante, Enzo, “Controlled Wellbore Implosions Show that Not All Damage is Bad—A New Technique to Increase Production from Damaged Wells,” SPE 144080, SPE European Formation Damage Conference, Netherlands, 7-10 Jun. 2011. When used with perforating guns for transient underbalance cleanup, these implosion chambers are typically initiated at the same time or within less than a millisecond of firing the perforating gun, all by firing a single detonator.